This invention relates to optical apparatus for scanning a beam of radiation over a surface. More particularly, this invention relates to rotary optical apparatus for continuously scanning radiation, particularly coherent radiation, over a surface as that surface is advanced through the path of the radiation.
There are a myriad of devices for optically scanning a beam of radiation. Typically, such apparatus either scan the radiation by displacing a reflector in a plane at a fixed angle or provide an arcuate scan by rotating the reflector. U.S. Pat. No. 3,769,488 discloses a laser cloth cutting system in which a beam of coherent radiation is scanned over a cloth cutting area by planar x-y displacement of a pair of optical reflectors. The coherent radiation could be scanned by use of a rotating reflector but the problem of maintaining focus of the radiation on the cloth is difficult and requires even more complex mechanisms.
Regardless of whether the scanning reflector is displaced in a plane or rotated, there are inherent problems which must be overcome. Apparatus for displacing the reflector in a plane must necessarily be operated intermittently as the amount of displacement has to be finite. This means that the system must provide means for accelerating the reflector to the operating velocity, decelerating it, reversing its direction and repeating the process. This necessarily presents significant design problems, especially as the demand for higher velocities and more accurate positioning have to be met.
The use of a continuously rotating scanning reflector does not provide a ready solution to the problem. The reflector will be inoperative during a significant portion of its revolution unless the surface being scanned is also circular. Multi-faceted reflectors can be used, but they are expensive and difficult to accurately construct even though they can be continously rotated. Moreover, they do not eliminate the focusing problem mentioned above. Maintaining focus is particularly critical when dealing with coherent radiation.
Significant improvements can and have been made in methods and apparatus for accurately scanning radiation by planar displacement of a reflector. For example, the apparatus disclosed in U.S. Pat. No. 3,769,488 has been improved by using linear induction motors operated under the control of a computer and digital circuitry. Velocities of 40 to 60 inches per second have been achieved. But the laws of physics inevitably limit the amount of improvement that can be obtained using this approach. Reducing mass to obtain greater acceleration and positional accuracy makes the system prone to vibration and other extraneous forces. Yet as lasers have become more powerful and reliable, there is a greater demand for higher scanning velocities and ever more accurate positioning of the beam of radiation.
The problem of scanning optical radiation necessarily involves considerations of energy control as well as the rate of delivery especially when the beam is coherent radiation being used to manufacture products by burning or ablating material from an object. Good resolution can be obtained at a high production rate when the spot size is small. This requires apparatus that maintains precise focus throughout the scan at spot sizes from, 0.001 inch down to 12 microns. This requires the use of lenses at low f numbers, such as f 0.5 when focusing coherent infrared radiation. Such lenses have short focal lengths and practically no depth of field. Consequently, the optical scanning apparatus must be capable of positioning the scanning lens close to the object and scanning without deviation from the scan path.
The present invention provides a solution to the problem of accurately scanning radiation, particularly coherent radiation of higher velocities and greater accuracy.
The reflectors are angularly spaced from each other and positioned radiallly outward from the axis of rotation by rigid arms or equivalent structure such as a disk.